The present invention relates generally to encased, electric motor driven devices and, more particularly, to a protection circuit for a fluid machine such as a hermetic compressor, having an internal electric motor drive. Fluid machines can be classified in many ways. By function they are classified as: (1) compressors, where the fluid machine acts on a gas to increase its pressure and reduce its volume, (2) pumps, where the fluid machine does not reduce the volume of the fluid in the device but moves the fluid from one place to another but may increase the pressure of the fluid in forcing it into the place of delivery; and (3) expanders, where pressurized fluid acts on the fluid machine causing it to move while the fluid increases in volume and decreases in pressure such as where steam acts on a turbine. A compressor and an expander are basically the same device with reversed operation. A pump and a compressor differ basically in whether or not the volume of the fluid changes in the device. The same basic structure can thus act as a pump, compressor or expander for some types of fluid machines.
Where classified by operation, a fluid machine is either centrifugal, in which velocity is converted to pressure, or positive displacement, where a volume of fluid is trapped and compressed, expanded or just moved. The positive displacement fluid machines are classified as reciprocating, as where there is a reciprocating piston acting in a cylinder, or as rotary. Rotary includes: (1) screw compressors, where the fluid is trapped in spaces partially defined by the lobes and grooves of the rotors, or, for a single screw compressor, where the fluid is trapped in spaces partially defined by the rotor, (2) scroll compressors, where the fluid is trapped in spaces partially defined between the wraps of the fixed and orbiting scroll members; (3) rotary vane, where a piston carrying radial vanes is rotated in a cylinder with the fluid being trapped between vanes; (4) fixed vane, or rolling piston, where the vane reciprocates to maintain sealing contact with an eccentric piston, with the piston contacting the wall of the cylinder and the vane to define the trapped volumes.
When classified by their housing or casing, fluid machines are classified as open drive, semi-hermetic and hermetic. In open drive machines, the crankshaft extends through the shell in which the portion of the fluid machine that acts on the fluid is located, but the driving means is located outside of the shell. In semi-hermetic machines the entire machine is within the shell but is field serviceable by the removal of sealed panels. Hermetic machines are entirely located within the shell which is welded together.
When classified by use, fluid machines are used either in an open system, as where air is the fluid and is exhausted to the atmosphere, or in a closed system such as one using Freon or some other refrigerant which is continuously circulated through the system during operation. In a closed system with a fluid machine acting as a compressor of a refrigerant, and having a hermetic casing or shell, the shell will be filled with refrigerant. If all or most of the shell is filled with refrigerant at suction pressure, then the compressor is referred to as a low side compressor, while if the shell is filled with refrigerant at discharge pressure, it is referred to as a high side compressor.
Because of shell strength requirements, hermetic compressors are limited in size and are typically used only in residential and light commercial air conditioning and refrigeration applications. The shell is penetrated by the suction and discharge lines and by the electric power input. This is the minimum number of penetrations and therefore the minimum sealing requirement. The interior of the shell normally has very little unused space available because of the desire to minimize the cubage, weight, material costs, and the amount of refrigerant within the shell.
Compressor protection is achieved by interrupting power to the compressor when it is operating under an undesirable condition. This protection is normally achieved by a stator mounted, thermally responsive switch located in the power line to the motor. This switch responds to excess motor temperature and/or an overcurrent by opening and thereby breaking the electrical circuit to stop the compressor.
There are, however, other conditions under which it is desirable to stop compressor operation in order to protect the compressor. These conditions include high discharge temperature, excessive pressure ratio, high superheat, and loss of charge. In scroll compressors, specifically, these conditions may produce an excessive thermal gradient in the scroll members between the inlet and outlet due to the heating of the refrigerant during compression. Also, a scroll compressor is capable of reverse operation wherein it acts as an expander when reverse flow is permitted, or as a vacuum pump if flow is blocked. A check valve in the discharge line prevents the reverse flow necessary for expander operation but traps a volume which is evacuated by the reverse operation of the compressor which acts as a vacuum pump because the reverse flow is limited to the fluid in the trapped volume. Each of the above-identified thermally abusive conditions can be sensed either directly or indirectly through a thermal sensor. Reverse rotation and excessive pressure ratio can also be sensed through pressure sensors. The sensing of the temperature of the compressor discharge gas or of the central portion of the fixed scroll of a scroll compressor will provide an indication of compressor operation problems that can be due to a number of conditions, as indicated above, requiring compressor operation to be stopped, although the specific cause for the temperature increase will not necessarily be known. The reverse direction of rotation of the motor of a scroll compressor can be sensed as a low pressure/vacuum at the compressor discharge where the supply flow is blocked and it becomes desirable to stop the compressor in response thereto. Major problems in sensing other conditions and in stopping the compressor in response thereto are: the limited internal space available, the problems with additional penetration of the shell, cost, and the need to power the sensors and/or responsive circuitry.